Since a technology called “Multiple-Input Multiple-Output (MIMO) plays an important role to increase a peak rate and a spectral efficiency for a system, standards for radio access techniques such as Long Term Evolution (LTE)/LTE-Advanced (LTE-A, i.e., a subsequent long term evolution) are established based on MIMO+OFDM (Orthogonal Frequency Division Multiplexing). A performance gain of the MIMO technique is from a spatial freedom degree obtainable from a multi-antenna system. Thus, a most important evolution direction in a standardization process of the MIMO technique is a dimension extension. In Release version 8 of the LTE, a four-layer MIMO transmission may be supported at most. In Release 9, a Multi-User MIMO (MU-MIMO) technique is enhanced specifically, and in a MU-MIMO transmission in a Transmission Mode (TM)-8, four downlink data layers may be supported at most. In Release 10, a spatial resolution of Channel State Information (the Channel State Information (CSI) may include a Channel Quality Indicator (CQI), a Precoding Matrix Indicator (PMI), a Rank Indication (RI)) is further increased by introducing an eight-port Channel State Information Reference Signal (CSI-RS), a User Equipment (UE)-specific Reference Signal (URS) and a multi-granularity codebook, and a transmission capability of a Single-User MIMO (SU-MIMO) is expanded to eight data layers at most.
In an antenna system of a base station utilizing a traditional Passive Antenna System (PAS), multiple antenna ports (each of the antenna ports corresponds to an independent Radio Frequency (RF)-Intermediate Frequency (IF)-Baseband channel) are arranged horizontally, and multiple oscillators in a vertical dimension corresponding to each of the ports are connected by a RF cable. Therefore, a relevant MIMO technique may only optimize a spatial characteristic of signals of each terminal in a horizontal dimension by adjusting a relative amplitude/a relative phase among different ones of the ports in the horizontal dimension, and may only use a unified sector-level forming in the vertical dimension. After an Active Antenna System (AAS) technique is introduced to a mobile communication system, the antenna system of the base station may obtain a larger freedom degree in the vertical dimension, and may optimize a signal at a UE level in a three-dimensional space.
Based on above research results, the standardization, and developments of antenna techniques, an industry is pushing the MIMO technique further forward in a direction of three-dimension and large scales. At present, 3GPP (3rd Generation Partnership Project) is engaged in a technical research and standardization of a Full Dimension MIMO (FD-MIMO). The academia is carrying out more forward-looking researches and tests for a MIMO technique which is based on larger-scale antenna arrays. A result based on academic researches and preliminary channel tests shows that a Massive MIMO technique may enhance a frequency-band utilization efficiency of a system significantly, and support a larger amount of users accessing to the system. Therefore, various research organizations take the Massive MIMO technique as a most prominent physical-layer technique in a next generation mobile communication system.
The Massive MIMO technique needs to use a massive antenna array. Although a fully digital array may maximize a spatial resolution and obtain a optimum MU-MIMO performance, such fully digital array needs a large quantity of A/D (analog/digital) converters and D/A (digital/analog) converters and a large number of complete RF-Baseband processing channels. Thus, a heavy burden exists in terms of device costs and baseband processing complexity. This problem is especially significant in a system configured with high frequency bands and larger bandwidths. In order to reduce the costs and complexity of the device using the Massive MIMO technique, a hybrid digital-analog beam-forming technique is given recently. The hybrid digital-analog beam-forming technique is a technique in which a stage of beam-forming processing is added to a RF signal at a front end near an antenna system based on a conventional digital beam-forming processing. An analog beam-forming processing may enable a rough match between a transmission signal and a channel in a simple manner. A dimension of equivalent channels formed after the analog beam-forming processing is smaller than an actual antenna amount, and thus the amount of A/D converters and D/A converters needed subsequently, a number of digital channels and baseband processing complexity may be greatly reduced. A residual interference in the analog beam-forming processing may be further processed in a digital domain, and thus a MU-MIMO transmission quality is ensured.
Compared with the fully digital beam-forming technique, the hybrid digital-analog beam-forming technique is a trade-off solution between a performance and the complexity, and has a higher application prospect in a system configured with high frequency bands, large bandwidths or a large amount of antennas.
In the MIMO technique, especially in the MU-MIMO technique, a precision of channel state information obtained by a network side may directly affect an accuracy of a precoding/beam-forming process and an effect of a scheduling algorithm, and thereby affect an overall performance of a system. Therefore, obtaining the channel state information is one of core issues in the standardization of the MIMO technique.
According to a structure of a current LTE signal, since a reference signal is arranged in the baseband, a channel state needed by the digital beam-forming may be obtained through channel estimation. However, since an amount of equivalent digital channels formed by the analog beam-forming is smaller than an actual antenna amount, a dimension of a channel matrix obtained through the reference signal is much less than a dimension of a complete channel matrix at antenna terminals. Therefore, the spatial resolution and an interference suppression capability achievable by the digital beam-forming are discounted. A processing of the analog beam-forming is closer to physical antennas, and thus MIMO channels of the analog beam-forming have a higher freedom as compared with those of the digital beam-forming. However, since the reference signal arranged in the baseband may not be estimated, the analog beam-forming in both an FDD (Frequency Division Duplex) system or in a TDD (Time Division Duplex) system may not directly use channel state information obtained in the digital domain.
Therefore, in a hybrid digital-analog beam-forming system, analog beams may generally be selected by a searching (or training) manner. In such selection procedure, a transmission end transmits a group of beams, a reception end also performs probing reception by using a group of predetermined beam so as to determine an optimum combination of transmission beams and reception beams. In a case that a channel condition is changed (such as a shielding happens), the system performs a beam searching procedure again, and a traversal search needs to be performed to possible combinations of transmission beams and reception beams.
For the hybrid digital-analog beam-forming system, a current beam searching and tracking procedure is performed basically in an analog domain and is used to select analog beams. A procedure of measuring and feeding back the channel state information in the digital domain is independent from that in the analog domain, and generally is performed after a training and tracking procedure of the analog beams, and measurement of the reference signal as well as the calculation and feedback of the channel state information are performed on an established combination of transmission beams and reception beams. Such two measurement and feedback mechanisms independent from each other and directed to the analog domain and the digital domain respectively bring large redundancy and system overheads.